1. Technical Field
This invention relates generally to ignitors used for igniting air/fuel mixtures in automotive application and the like, and in particular to a self-tuning power amplifier for use in a corona ignition system.
2. Related Art
U.S. Pat. No. 6,883,507 discloses an ignitor for use in a corona discharge air/fuel ignition system. According to an exemplary method used to initiate combustion, an electrode is charged to a high, radio frequency (“RF”) voltage potential to create a strong RF electric field in the combustion chamber. The strong electric field in turn causes a portion of the fuel-air mixture in the combustion chamber to ionize. The process of ionizing the fuel-air gas can be the commencement of dielectric breakdown. But the electric field can be dynamically controlled so that the dielectric breakdown does not proceed to the level of an electron avalanche which would result in a plasma being formed and an electric arc being struck from the electrode to the grounded cylinder walls or piston. The electric field is maintained at a level where only a portion of the fuel-air gas is ionized—a portion insufficient to create the electron avalanche chain reaction described previously which results in a plasma. However, the electric field is maintained sufficiently strong so that a corona discharge occurs. In a corona discharge, some electric charge on the electrode is dissipated through being carried through the gas to the ground as a small electric current, or through electrons being released from or absorbed into the electrodes from the ionized fuel-air mixture, but the current is very small and the voltage potential at the electrode remains very high in comparison to an arc discharge. The sufficiently strong electric field causes ionization of a portion of the fuel-air mixture to facilitate the combustion reaction(s). The ionized fuel-air mixture forms a flame front which then becomes self-sustaining and combusts the remaining fuel-air mixture.
FIG. 1 illustrates a capacitively coupled RF corona discharge ignition system. The system is termed “capacitively coupled” since the electrode 40 does not extend out of the surrounding dielectric material of the feedthru insulator 71b to be directly exposed to the fuel-air mixture. Rather, the electrode 40 remains shrouded by the feedthru insulator 71b and depends upon the electric field of the electrode passing through part of the feedthru insulator to produce the electric field in the combustion chamber 50.
FIG. 2 is a functional block diagram of the control electronics and primary coil unit 60 according to an exemplary embodiment of the invention. As shown in FIG. 2, the control electronics and primary coil unit 60 includes a center tapped primary RF transformer 20 which receives via line 62 a voltage of 150 volts, for example, from the DC source. A high power switch 72 is provided to switch the power applied to the transformer 20 between two phases, phase A and phase B at a desired frequency, e.g., the resonant frequency of the high voltage circuit 30 (see FIG. 1). The 150 volt DC source is also connected to a power supply 74 for the control circuitry in the control electronics and primary coil unit 60. The control circuitry power supply 74 typically includes a step down transformer to reduce the 150 volt DC source down to a level acceptable for control electronics, e.g., 5-12 volts. The output from the transformer 20, depicted at “A” in FIGS. 1 and 2, is used to power the high voltage circuit 30 which is housed in the secondary coil unit, according to an exemplary embodiment of the invention.
The current and voltage output from the transformer 20 are detected at point A and conventional signal conditioning is performed at 73 and 75, respectively, e.g., to remove noise from the signals. This signal conditioning may include active, passive or digital, low pass and band-pass filters, for example. The current and voltage signals are then full wave rectified and averaged at 77, 79, respectively. The averaging of the voltage and current, which removes signal noise, may be accomplished with conventional analog or digital circuits. The averaged and rectified current and voltage signals are sent to a divider 80 which calculates the actual impedance by dividing the voltage by the current. The current and voltage signals are also sent to a phase detector and phase locked loop (PLL) 78 which outputs a frequency which is the resonant frequency for the high voltage circuit 30. The PLL determines the resonant frequency by adjusting its output frequency so that the voltage and current are in phase. For series resonant circuits, when excited at resonance, voltage and current are in phase.
The calculated impedance and the resonant frequency are sent to a pulse width modulator 82 which outputs two pulse signals, phase A and phase B, each having a calculated duty cycle, to drive the transformer 20. The frequencies of the pulse signals are based on the resonant frequency received from the PLL 78. The duty cycles are based on the impedance received from the divider 80 and also on an impedance setpoint received from a system controller 84. The pulse width modulator 82 adjusts the duty cycles of the two pulse signals to cause the measured impedance from the divider 80 to match the impedance setpoint received from the system controller 84.
The system controller 84, in addition to outputting the impedance setpoint, also sends a trigger signal pulse to the pulse width modulator 82. This trigger signal pulse controls the activation timing of the transformer 20 which controls the activation of the high voltage circuit 30 and electrode 40 shown in FIG. 1. The trigger signal pulse is based on the timing signal 61 received from the master engine controller 86, not shown. The timing signal 61 determines when to start the ignition sequence. The system controller 84 receives this timing signal 61 and then sends the appropriate sequence of trigger pulses and impedance setpoint to the pulse width modulator 82. This information tells the pulse width modulator when to fire, how many times to fire, how long to fire, and the impedance setpoint. The desired corona characteristics (e.g., ignition sequence and impedance setpoint) may be hard coded in the system controller 84 or this information can be sent to the system controller 84 through signal 63 from the master engine controller 86. The system controller 84 may send diagnostics information to the master engine controller 86, as is customary in modern engine controls and ignition systems. Examples of diagnostic information may include under/over voltage supply, failure to fire as determined from the current and voltage signals, etc.